A wide variety of computerized systems, from the smallest personal digital assistants to the most powerful supercomputers, use memory to store programs for fast execution, and to store data for rapid access while the computer system is operating. Volatile memory, such as the dynamic random access memory (DRAM) most commonly found in personal computers, is able to store data such that it can be read or written much more quickly than the same data could be accessed using nonvolatile storage such as a hard disk drive or flash nonvolatile memory. Volatile memory loses its content when power is cut off, so while it is generally not useful for long-term storage, it is generally used for temporary storage of data while a computer is running.
A typical random-access memory consists of an array of transistors or switches coupled to capacitors, where the transistors are used to switch a capacitor into or out of a circuit for reading or writing a value stored in the capacitive element. These storage bits are typically arranged in an array of rows and columns, and are accessed by specifying a memory address that contains or is decoded to find the row and column of the memory bit to be accessed.
The memory in a computer usually takes the form of a network of such circuit elements formed on an integrated circuit, or chip. Several integrated circuits are typically mounted to a single small printed circuit board to form a memory module, such as single inline memory modules (SIMMs) having a 32-bit memory channel for reading and writing data, or dual inline memory modules (DIMMs) having a 64-bit memory channel. Some more sophisticated types of memory modules include synchronous dynamic random access memory, or SDRAM, which runs in synchronization with the computer's bus, and double data rate (DDR) SDRAM or DDR2 SDRAM, which transfer data on both the rising and falling edges of the clock and have memory channel widths up to 64 bits of data and 8 bits of error management information per memory transfer. DDR2 SDRAM is a random access memory technology used for high speed storage of the working data of a computer or other digital electronic device.
Improvements in memory technology over time include making memory chips smaller, faster, and operable to consume less power and therefore to generate less heat. But, the constant push to improve memory performance and the imperfect nature of manufactured goods in general suggest that occasional flaws or imperfections will occur. Individual memory bit storage locations occasionally go bad, and sometimes even whole memory chips fail. It is also known that various electrical phenomena can regularly cause memory read or write errors, such as electromagnetic noise causing a signal level to change or a cosmic ray changing the state of one or more bits of memory. Reductions in the size of memory elements and reductions in the voltage used to operate the memory make such problems increasingly important to consider when designing memory.
Error management is therefore implemented in many memory systems, and is most typically embodied in a single parity bit per data byte that is operable to indicate when a single bit has changed state, or error correction codes (ECC) that can detect and often correct single-bit errors in memory systems. Even though the reliability of individual memory components is very high, the number of memory components in large computer systems and the cost involved with producing the amount of memory needed make memory error detection and correction an important consideration in memory system design.